1. Field of the Invention
The present invention relates to a hydraulic circuit for a boom cylinder combination having a float function which is capable of implementing a leveling work in such a manner that a leveling work is performed by lowering a boom based on its self-weight without using an operation oil discharged from a hydraulic pump using an excavator during a leveling work, and in particular to a hydraulic circuit for a boom cylinder combination having a float function which is capable of implementing a leveling work in such a manner that a leveling work is performed by lowering a boom by its self weight by combining an operation oil of a head side and a rod side of a boom cylinder to a hydraulic tank without using an operation oil.
2. Description of the Background Art
As shown in FIG. 1, a caterpillar type excavator includes a lower running body A which runs when a running motor is driven, a upper rotation body D which is engaged rotatably in the lower running body A in a let and right direction and on which a driving room B is formed, a boom E which has an end rotatably engaged to the upper rotation body D and which is driven when a boom cylinder f is driven, an arm H which has one end rotatably fixed to the other end of the boom E and which is driven when an arm cylinder G is operated, and a bucket J which is fixed to the other end of the arm H rotatably based on a link movement and which is driven when the bucket cylinder I is driven.
Generally, when a leveling work is performed using an excavator, a floating function is adapted, so that a leveling work is effectively performed based on a curve surface in a work ground. In particular, without using an operation oil discharged from a hydraulic pump, an operation of a head side and a rod side of a boom cylinder is combined, and a boom is guided to be lowered by its self weight using a hydraulic pump for thereby implementing a leveling work. Therefore, the operation oil discharged from the hydraulic pump may be used for other work apparatuses, so that it is possible to save energy.
FIG. 2 is a cross sectional view illustrating a control valve for a boom cylinder combination having a float function in a conventional art.
As shown therein, the control valve for a boom cylinder combination in the conventional art includes a boom cylinder f which is connected with a hydraulic pump P and operates when an operation oil is supplied, a valve block c in which a spool is slidably installed for controlling a driving, stop and direction change of the boom cylinder and which is installed in a flow path between the hydraulic pump P and the boom cylinder f in such a manner that the valve block is switched when a pilot signal pressure Pi is applied, and a cap (a) which is engaged at one end of the valve block c and has a port into which a pilot signal pressure is applied for switching a spool d and has an elastic member b which is capable of returning the spool d to its initial position when a pilot signal pressure is released.
In the drawings, reference character “e” represents a high pressure path in which a high pressure oil is supplied from a hydraulic pump P to a small chamber g of the boom cylinder f through a flow path r, and p represents a low pressure path for guiding an operation oil from the large chamber I and the small chamber g of the boom cylinder to the hydraulic tank T through the flow path m, r.
Therefore, as shown in FIGS. 1 and 2, when the leveling work is performed in such a manner that the boom is lowered so that the bucket of the excavator contacts with the ground, as the operation lever which is provided for a leveling work is operated, the boom down pilot signal pressure Pi is supplied to the interior of the cap a formed in a right end of the valve block c, and the spool d installed in the valve block c is switched in the left direction, overcoming the elastic force of the elastic member b installed in the cap a, and a high pressure operation oil flows through the paths e, q, and r, in sequence, and is supplied to the small chamber g of the hydraulic cylinder f.
At this time, the operation oil in the side of the small chamber g of the boom cylinder is connected with the hydraulic tank T through the path r, the pockets I, j of the valve block c, and the low pressure path k, and the operation oil in the side of the large chamber I is connected with the hydraulic tank T through the pockets n, o formed in the path m and the valve block c and through the low pressure path k.
Therefore, in order to implement a leveling work, lowering the boom of the excavator, the operation oil discharged from the hydraulic pump P is used. In this state, the leveling work is performed by connecting the operation oil in the sides of the head and rod is connected to the hydraulic tank T. In this case, it is impossible to obtain an energy saving effect which is one of the major reasons when using the float during the leveling work. In addition, in the conventional art, since the valve bock having a float function is additionally provided with respect to the main control valve, the number of parts is increased, and the fabrication cost is increased.